The present invention relates to ohmic contacts to silicon devices and more particularly to plated ohmic contacts.
Heretofore, plated ohmic contacts to some silicon devices have been made by plating the silicon with a layer of nickel. Nickel is used bacause when nickel is sintered at a temperature range of about 575.degree.C to 625.degree.C for about 10-30 minutes in a reducing atmosphere, such as 10% H.sub.2 and 90% N.sub.2, nickel will alloy with the silicon to form a strong bond. Sometimes a higher temperature, such as 800.degree.C, is also used.
The alloying of nickel and silicon, however, results in nickel silicide surface which is only partially metallic in nature. Poor mechanical bonds are established between thermal or electrical leads and the sintered nickel layer, i.e. thermal or electrical leads soldered to the sintered nickel layer do not form high strength bonds. Thus, frequently a second layer of metal is plated over the sintered nickel layer, to protect the sintered nickel layer from oxidation and to form a layer to which thermal or electrical leads can be soldered. In addition, the second layer of metal must form a strong bond with the sintered nickel layer in order to insure a mechanically strong contact.
Gold has been used as the second layer of metal (see e.g. U.S. Pat. No. 3,169,304). Gold will alloy with the sintered nickel at a temperature of above about 300.degree.C to form a strong bond. This temperature can be achieved when thermal or electrical leads are subsequently soldered onto the layer of gold. LThe heat of the soldering operation is sufficient to strengthen the sintered nickel-gold bond. However, at comparable thickness, gold is more porous than other metals, such as cobalt. Thus, a relatively thick and stable layer of gold must be applied to the sintered nickel layer to prevent the sintered nickel layer from oxidation. A relatively thick and stable layer of gold is difficult to apply and is expensive.